1. Field of the Invention
The present invention relates generally to differential support structures, differentials' components, methods of manufacturing differential support structures, and methods of manufacturing differentials' components, and particularly to differential support structures, differentials' components, methods of manufacturing differential support structures, and method of manufacturing differentials' components that allow increased life against fatigue.
2. Description of the Background Art
Deep groove ball bearings, tapered roller bearings and other similar rolling bearings are formed of rolling elements, bearing rings and cages. When the rolling elements roll between the bearing rings, the rolling elements contact the bearing rings at a point or linearly. As such, while the bearing's projected area is small, a large load bearing capacity and high rigidity can advantageously be obtained. As such, rolling bearings are suitable for support structures used under severe conditions such as reduced lubrication, operation at high rotation rates, and the like, and for example used in a structure supporting an automobile's differential.
In such an automobile's differential support structure the bearing's components are fatigued as they rotate. To increase the components in life, a thermal treatment is performed. Specifically, for example, in quenching the components they are heated in an ambient RX gas with ammonium gas further introduced therein to carbo-nitride their surface layer portion, for example as disclosed in Japanese Patent Laying-Open Nos. 8-4774 and 11-101247. This carbo-nitriding process can harden the surface layer portion and generate retained austenite in a microstructure to provide increased rolling contact fatigue life.
However, an automobile differential support structure carries radial load based mainly on gravity. Furthermore in the automobile differential support structure a pinion gear and a side gear are prevented from meshing at a varied position by exerting a considerable radial load on rolling elements supporting the differential. Furthermore in the automobile differential support structure the pinion and side gears fit with a helix angle. This results in thrust load. Furthermore to allow the pinion and side gears' fitted portion to be quiet a considerable thrust preload is exerted on the rolling elements supporting the differential. These loads are combined and the rolling elements supporting the differential carry a considerable thrust load. In order to obtain sufficient rolling fatigue characteristic the structure needs to have a sufficient large size, which is an obstacle to reducing the differential in size.
Furthermore, the above-mentioned carbo-nitriding process is a process to diffuse carbon and nitrogen. This requires a high temperature maintained for a long period of time. As such, for example a coarsened structure results and increased anti-crack strength is hardly obtained. Furthermore, as more austenite is retained, secular dimensional variation rate increases, which is also an obstacle to providing increased life.
Against rolling fatigue, an increased life can be ensured, an enhanced anti-crack strength provided and an increased secular dimensional variation avoided by relying on designing a steel alloy to provide an adjusted composition. Relying on designing the alloy, however, increases source material cost disadvantageously.
As future bearings will be used in environments exerting large loads at high temperatures, the bearings will be required to be operable under larger loads at higher temperatures than conventional. As such, there is a demand for a bearing having large strength, long life against rolling contact fatigue, and large anti-crack strength and dimensional stability. Furthermore there is also a demand for not only a bearing component in a differential support structure but also a gear, a shaft and other components of the differential that allow long life against fatigue.